1. Field of the Invention
The present invention relates to an optical disc reproducing apparatus, and more particularly, to a device for detecting a digital bit in an optical disc reproducing apparatus, which can minimize occurrence of errors in reproducing a digital bit from a radio frequency(RF) signal received at the optical disc reproducing apparatus.
2. Discussion of the Related Art
A general optical disc, such as a compact disc, digital audio disc and digital video disc, stores digital bit data of "1" or "0" in forms of pits which vary a reflection ratio or a refractive index of a light beam. These pits, formed by a light beam of relatively high energy, compose tracks on an information recording surface of the optical disc. In order to prevent the light beam from being off the track due to excessive direct current component generated during a reproduction, the digital bit data is formatted such that logical values of "1" or "0" are not in succession more than a certain times, for example, in the case of 8-14 modulated data, no more than 3 to 11 times so that the RF signal picked up from the optical disc by an optical pick-up is always within a specified range of width. The specified range of width herein is 3T to 11T.
In the meantime, the optical disc can not be formed to have a uniform plane, but is formed to have a slight asymmetrical plane due to a mechanical tolerance. And, spots of a light beam directed onto a surface of the optical disc can not be made always to have a fixed size and form due to a mechanical tolerance of the optical pick-up, too. Because of these reasons, the width of the RF signal picked up from the optical disc by the optical pick-up either exceeds or falls short of the specified range to occur an error in a digital bit.
However, it is a current trend to form the pits as closer as possible on the optical disc for recording a larger amount of data. But this high concentration of pits causes interferences between the pits, accelerating the problem of noise component occurrence in the RF signal and acting as a cause increasing an error occurrence rate in a digital bit data. In order to minimize such error occurrence in the digital bit data, a digital bit data reproducing device employs a method in which the RF signals are adaptively interpolated according to a transmission rate of the RF signal. However, because of the asymmetry of the optical disc and the focusing errors of the light beam, this adaptive interpolation method could not have been possible to correct distortions of the RF signals coming from the interference, though it could have been possible to remove relatively small noise components. Because of this, the digital bit reproducing device for an optical disc has a problem in that an amount of error in the digital bit data can not be reduced below a certain limit.
In order to solve the aforementioned problems, the same applicant has applied the Korean Patent No.95-55629 related to a device for reproducing a digital bit in an optical disc reproducing apparatus, which will be explained briefly, hereinafter.
In the device for reproducing a digital bit in an optical disc reproducing apparatus by the applicant, a RF signal reproduced from an optical pick-up is converted into a digital RF data, subjected to adaptive interpolation according to a transmission rate, and corrected of an asymmetry error of the optical disc. The RF data having corrected of the asymmetry error is taken of its absolute value to restore the digital bit. In this RF data adaptive interpolation method, a clock signal corresponding to the transmission rate of the RF data is generated using a digital synchronous loop, and a logical value of the RF signal is adjusted at fixed phases of the clock signal. This adaptive interpolation method makes interpolation of the RF data bit by bit, allowing removal of a momentary excessive noise, i.e., a noise component coming from a focusing error of the light beam, but not allowing the correction of a RF data distortion coming from interferences between pits; a width of the RF signal that falls short of the specified range can not be corrected to be within the specified range and the asymmetry correction to be within the specified range is also not possible though removal of the momentary noise component coming from a tilting of the optical disc is possible. Because of this, the conventional digital bit reproducing device in an optical disc reproducing apparatus has a problem in that occurrence of errors in a digital bit data can not be reduced below a certain limit because the correction of the distortion of the RF signal coming from interferences between pits in the optical disc is not possible.